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I am working on a project where I would like to power an RGB Led in a ambient light situation. I have prototyped this using an Arduino Leonardo and the led. Currently the project is plugged into the USB port of my computer, however I would like to use an Attiny 45 or 84 and AA batteries. I was originally thinking of using 3 AA non rechargeable batteries which would provide about 4.8 volts when fully charged. The issue is that the voltage will drop over time and I would like the led to be a constant brightness. A voltage regulator came to mind, but it would need to be efficient since I would like the batteries to last quite a while. I looked at some but was not sure the best one for my situation.

The led I am using is from SparkFun - SparkFun RGB led.

Another route would be to use two AA batteries and a step up board like is here SparkFun 3.3v breakout but again battery life is in question. I am not sure how to calculate battery usage through a step up board.

Long battery life in this situation means it would last for weeks, maybe months, of on and off use. I may be asking for the impossible with only 2 or 3 batteries though.

So my questions are would one of these options be better than the other? Would the brightness not change that much with 3 AA batteries and no voltage regulator? Would the batteries last a long time with the second option? Is there another option I have missed?

I am new to the hardware side of things, so any help is much appreciated!

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    \$\begingroup\$ It's a shame no answers yet address the question of calculating power needs of Attiny45 and step-up regulator etc. If only to explain why it isn't relevant, or not possible, to do so. \$\endgroup\$ – RedGrittyBrick Sep 10 '13 at 8:39
  • \$\begingroup\$ I am really sorry your answer was spammed, we have a site troll and he seems to have taken interest in your question. \$\endgroup\$ – Kortuk Sep 10 '13 at 13:30
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    \$\begingroup\$ Yes I would appreciate if someone could explain the power usage, even if it's only to prove you can't calculate that. Thank you to whoever removed those other troll replies. \$\endgroup\$ – Elmer Sep 10 '13 at 13:55
  • \$\begingroup\$ The ATtiny45 can run on as little as 2.7V, so you should be able to run it directly off the batteries. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 10 '13 at 13:58
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Constant LED brightness means constant current, and constant current means a current limiter.

schematic

simulate this circuit – Schematic created using CircuitLab

Select R2 as (VD2 + VD3 - VbeQ1)/ID1. Don't be afraid to use a current slightly lower than the spec (i.e. 3 to 6mA).

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    \$\begingroup\$ And of course you went and picked a CC LED. FFFFFFFFFFFFFFFF..... \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 10 '13 at 5:24
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    \$\begingroup\$ To clarify Ignacio's comment: The Sparkfun RGB LED has three LEDs with a common cathode (CC). Ignacio's circuit is not suitable for use with that LED. I believe Ignacio's circuit is a variant of this \$\endgroup\$ – RedGrittyBrick Sep 10 '13 at 8:37
  • \$\begingroup\$ Sparkfun does sell CA LEDs, but that involves an additional acquisition. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 10 '13 at 13:59
  • \$\begingroup\$ I thought common anode and common cathode LEDs worked the same, just whether you pulled the pin HIGH or LOW? \$\endgroup\$ – Elmer Sep 10 '13 at 17:11
  • \$\begingroup\$ @Elmer: Yes, except that using a CA LED will allow you to create 3 copies of the circuit in parallel, and control them individually. With CC you need to use a high-side limiter, and the control circuitry for that is a little more complex. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 10 '13 at 17:23
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Find the datasheet for your cells like this example. Then take a look at the discharge curve and check how battery voltage changes during the discharge cycle. Then check how your LED responds to that using a bench power supply.

Couple things you can do from that moment:

  • Determine at which point brightness has dropped too much and if it is worth to consider the cell empty anyway;
  • Determine maximum and minimum supply voltage and use that requirements to make a better regulated power supply;
  • Increase number of batteries in series, and increase series resistor. The higher the series resistor, the lower response to varying supply voltage, but losses increase.
  • Use a current sink like proposed in one of the other answers.
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protected by Kortuk Sep 10 '13 at 13:27

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